A typical fluorescent lamp includes a sealed glass tube containing a rare earth gas, and an electrode at each end for striking and maintaining an electric arc through the gas. The electrodes are typically constructed as filaments to which a filament voltage is applied to heat the electrodes, thereby improving their capability to emit electrons. This results in improved electric arc stability and longer lamp life.
Typical prior art ballasts apply the filament voltage to the filaments prior to striking the arc and maintain the filament voltages throughout the entire dimming range of the lamp. At low end, when light levels are lowest and, consequently, the electric arc is at its lowest level, the filament voltage helps maintain a stable arc current. At high end, when light levels are highest, and the electric arc current is at its highest level, the electric arc current contributes to heating the filaments.
FIG. 1 is a perspective view of an example gas discharge lamp fixture 100. The fixture 100 includes a ballast 102, lamp sockets 104, and a housing 106. The ballast 102 and the sockets 104 may be fixed to the housing 106. The lamp sockets 104 may be sized and situated within the housing 106 to hold lamps 108. The ballast 102 may have wires 110 to connect the ballast 102 to the sockets 104 for driving the lamps 108 and for providing heating current, discussed above. In practice, the ballast 102 may be wired by a fixture supplier, as is common in new construction, or it may be wired by an on-site installer, as is common in retrofit projects.
A typical ballast may be configured for operation with a particular lamp type, for instance, a lamp from a particular manufacturer. The ballast may be configured, for example, at the time the ballast is manufactured or at the time the ballast is installed. For example, a ballast may be configured to apply a pre-determined filament voltage to the lamps, such that that the filaments of each lamp will be heated in accordance with an optimal range of filament heating temperatures associated with that lamp type. If too much or too little filament voltage is applied, the filaments of the lamps may be under heated or overheated, which may detrimentally affect the lamps, for example, by shortening the useful lives of the lamps.
Lamps from different manufactures typically have different optimal ranges of filament heating temperatures. Accordingly, if a ballast is configured to operate with a first lamp type, and then actually operated with a second lamp type, the ballast may not heat the filaments of the second lamp type within the optimal range for that lamp type.